This invention relates to a process for preparing M' niobium activated yttrium tantalate x-ray phosphor having an M' crystal structure by a method in which a reduced amount of flux is used during milling, reducing or eliminating crucible corrosion while improving phosphor brightness. The phosphor is essentially M'YTaO.sub.4 and exhibits improved luminescent properties compared to phosphors produced using greater amounts of flux. Also, reduced amount of Nb.sub.2 O.sub.5 activator results in equivalent or improved brightness.
X-ray phosphors are used in x-ray intensifying screens which are used along with photographic film to enhance the photographic image formed on the film at the same time reducing the x-ray dose on the object during medical radiographic procedures. Phosphor materials used in these intensifying screens are to be colorless single phase with a polyhedral shape of well-defined crystal morphology. Also, the phosphors have to be good x-radiation absorbers and emit the light (energy) in the spectral region to which the photographic film is sensitive. Generally, it is required that the phosphor particle size be about 4-11 micrometers in order to form a thin layer when drawn in the form of screens using certain binder solutions as media. The phosphor material also has to have a high x-ray energy absorbing property. After absorbing the x-ray energy, when exposed, the phosphor should emit photons (light) strongly in the spectral region of the film sensitivity. The efficiency of x-ray energy-to-light conversion should be intense enough to obtain undistorted and sharp film images. There are several materials of such kind but only few have good properties necessary to make them as useful materials for intensifying screen applications.
Blasse and Bril (J. Luminescence, 3,109 (1970) describes the cathodo-and-photo luminescence properties of various rare earth tantalate phosphors. These materials have fergusonite (M-type) monoclinic crystal structure. Wolten & Chase (American Minerologist, 52, 1536 (1967)) report that this type of tantalate (e.g., YTaO.sub.4, and other rare-earth tantalates) has two polymorphs, a monoclinic (1.sub.2 Space group) structure-M at low temperature and a tetragonal (Scheelite type structure with 1.sub.41/a space group) at high temperature. Crystal structure transition between these two forms occurs at 1325.degree. in YTaO.sub.4 and is reversible. They disclose also the formation of a new polymorph of ytrrium tantalate and other rare earth tantalates. This new polymorph is obtained when the tantalates are synthesized (crystallized) below the above mentioned (1325.degree.C.) transformation and this polymorph has a monoclinic structure with P.sub.2/a space group which is called M' phase. M' phase can be converted to M phase by heating above 1400.degree. C. and then cooling to below the transition (1325.degree. C.) temperature.
Brixner & Chen (J. Electrochemical Soc., 130 (12), 1983, 2435-43) and U.S. Pat. No. 4,225,653 describe the preparation and the crystal structure of M' phase rare earth tantalate materials and their luminescence properties. They also demonstrate that the M' phase YTaO.sub.4 is an efficient host for x-ray phosphor when activated with niobium and some rare earth ions. However, it has been found that the preparation procedure is critical to obtain a single phased M'-YTaO.sub.4 with increased brightness when activated with niobium. Brixner & Chen recommend the preparation of niobium activated M' rare earth tantalate phosphor by pre-firing the component oxides TaO.sub.5, NbO.sub.5, and Ln.sub.2 O.sub.3 (Ln=La, Y, Ce, and Lu) at 1200.degree. C. for 8-10 hours. The reaction products are then milled using Freon solvent as grinding fluid for about 6 hours using alumina beads as grinding medium. The resulting mixture is then either alone or with 50% by weight lithium sulfate as flux material, fired at 1250.degree. C. for 10-14 hours. This procedure is cumbersome and the freon used in milling is expensive and environmentally hazardous.